Some observations on the nature of aerial survey

FROM THE AIRUNDERSTANDING AERIAL ARCHAEOLOGY

KENNETH BROPHY AND DAVID COWLEY

First published

Tempus Publishing LimitedThe Mill, Brimscombe Port,Stroud, Gloucestershire, GL5 2QGwww.tempus-publishing.com

© Kenneth Brophy and David Cowley,

The right of Kenneth Brophy and David Cowley, to be identified as the Authors of this work has been asserted in accordance with the Copyrights, Designs and Patents Act 1988.

All rights reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without the permission in writing from the Publishers.

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isbn 0 7524 3130 7

Typesetting and origination by Tempus Publishing LimitedPrinted in Great Britain

This volume is dedicated to Gordon Maxwell. As the pioneer of aerial survey at RCAHMS he has had a dramatic impact on the archaeological record in Scotland across

a number of different fields, most notably Roman Scotland. He is responsible for the identification of many new types of site amongst the cropmarks and has played a pivotal

role in putting in place the building blocks of so much of our understanding of the archaeology of lowland Scotland. He has also fostered and trained many other workers,

ourselves included. A simple query of Gordon, would so often (usually!) end up as a master class in aerial photographic interpretation, where the points raised by way of digression

and amplification alone enriched your experience and knowledge of the material – shared as ever with enthusiasm and kindness.

His impact on the editors, within and beyond RCAHMS, made this volume possible.

Kenneth Brophy and David Cowley, January 2005

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Some observations on the nature of aerial survey

David C. Cowley & Simon M.D. Gilmour

INTRODUCTION

The processes involved in survey of any type are manifold and complex and range across the technical, intellectual and emotional. Where they have been articulated, discussion has tended to concentrate on the mechanics of the procedures and technical aspects of equipment. Much more difficult to express are the intertwined roles of perception, experience and cognition, which operate at an individual level and render the survey process an essentially subjective experience, driven by a cycle of decision making, both conscious and unconscious. This subjectivity is increasingly recognised and can be used as a facet of survey, both as a means of understanding what elements of the products of survey are a consequence of the individual(s) undertaking the work or the methodologies employed, and as a mechanism to engage in a reflective, or ‘reflexive’, way with these processes and the material under examination. In this context the assessment of subjectivity and bias in survey should begin to shrug off the negative connotations that these terms often evoke (cf. Brophy this volume), enabling survey practitioners and end-users to engage in dialogue about this sometimes vexed issue. Aerial survey is no different in this respect, and there is a long-standing recognition of the pervasive subjectivity of the processes involved, from planning, through implementation, to analysis of aerial-derived material (e.g. Brophy ; Cowley ; Featherstone & Bewley ; Palmer ; Raçzkowski ). However, despite this interest there is surprisingly little published material describing the detail of the survey process beyond the generalities of, for example, the circumstances that lead to cropmark formation; ranging across issues such as where to look and the screening and evaluation of patterns. This paper explores

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some of these issues, highlighting the potential value of ‘flight biographies’ in conveying this type of information, and presenting a case study of the discovery during aerial survey of a pit-defined cursus monument in Fife during July , which is placed in the wider context of RCAHMS’s national summer flying programme. In this paper aerial survey is used to refer to ‘observer-directed’ reconnaissance and photography (a term coined by Rog Palmer), as opposed to block coverage (more usually referred to using the shorthand of ‘verticals’).

FLIGHT BIOGRAPHIES

The credit for the innovation of applying aerial reconnaissance to archaeological subjects in Scotland can be credited to O.G.S. Crawford, who undertook a groundbreaking series of flights in the s, publishing his results and observations in Antiquity (; ). In each of these published contributions he adopts a personal style, describing the work as a narrative (a term he applied to his publication (, )). This narrative highlights the very personal decisions made and their impact on choices of target and route, the thought processes involved in recognition and interpretation of features, his emotional state, ranging from anxiety to elation, and the excitement of discovery. Crawford’s biographical approach to disseminating the results of his flights is interesting because it incorporates many details on the reasoning and practice of recording from the air that provide the context for the subsequent flights from which photographs were published. In fact, Crawford apologises for the, ‘slight but inevitable intrusion of the personal element imposed by the character of the narrative’ (, ). Details such as the fact that the pilot and observer were able to sit side by side, and the advantages of this disposition in the plane, as well as the fact that sites were being spotted by both the pilot and the observer, and that the flight path followed Roman roads whenever possible, are useful and important observations. Descriptions include what was not recorded as well as what was, the emotional impact of the discoveries made and even admit that without a comparable ground survey much may have been missed even along such well-known features as Hadrian’s Wall, or that features may need to be ‘ground-truthed’ as at Burnswark, Dumfriesshire (ibid., -). Discussion of the weather and its impact on the flights, and especially the combination of weather and topography in Scotland during flights undertaken to accompany the production of the Ordnance Survey Map of Roman Britain (Crawford , ), give flesh to the bare bones of flight routes. During these later flights it was again the pilot who is credited with discovering many of the new sites and in fact took all the photographs (ibid., ). The process of interpretation in-flight is briefly addressed in the context of recognising both Roman signal stations from relict sheepfolds (ibid., ) and the presence of Roman roads as revealed by the pattern of their quarry pits (ibid., ). Crawford also noted sites that would repay

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FROM THE AIR

further flights during different conditions and areas where, had more time been available, more sites than those recorded might be discovered (ibid., ). The importance of this flight biography style of recording aerial reconnaissance is made clear when Crawford described the discovery of Cardean Roman fort () as partially accidental because they were looking for a different site at the time (ibid., -). He is also candid about his aims, observing that they had, ‘tried not to neglect entirely the remains of other periods. Native forts were so numerous that is was difficult, and sometimes impossible, to check them on the map’ (ibid., ; see also Jones this volume); how, one wonders, would such frankness be received today? These ‘personalised’ descriptions of flights allow us to understand and evaluate the context of the material discovered and recorded. Incidentally, another pioneer of Scottish aerial reconnaissance, J.K.S. St Joseph, also compiled detailed (unpublished) flight logs and these are preserved in The Unit for Landscape Modeling at Cambridge (formerly CUCAP). These specify

The Roman fort at Cardean as seen from the air in . The site was discovered, partly by accident, from the air by O.G.S. Crawford. Crown Copyright: RCAHMS, AN

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the motivations of sorties and the circumstances surrounding the development of the flight, what was photographed or not photographed, and why. St Joseph’s motivation in producing these logs included conveying what he did and why to workers using his material (D.R. Wilson pers. comm.). Publication of aerial reconnaissance since Crawford has invariably concentrated on the interpretation of the imagery itself. Discussion focuses on various factors that influence the pictures and are what might be described as physical factors, generally out of the control of the photographer, such as geology and geography, weather, visibility and cloud cover, as well as physical factors much more within human control, such as the type of aircraft, camera and film. However, all of these are placed in a formal introduction to the interpretation and, more recently, mapping of the results, rather than a detailed analysis of the survey process itself, and the less tangible factors that impact on the results of any given sortie. For example, the introduction (and these discussions are often limited to the introduction, preface or first chapter of any publication) to The Uses of Air Photography second edition concentrates on the various film types, the types of photographic cover available, including the various scales, and resolution of vertical versus oblique photography (St Joseph , -). There then follows information on the geology, geography, visibility and cloud cover that might affect aerial photographs (ibid., ). The closest this publication comes to discussing the process of the flights themselves is an analysis of the difference between targeted and general flying, the former the equivalent to oblique photography and the latter of vertical photography (ibid., ). Derek Riley covers some of the same issues in his Early Landscapes from the Air but also goes into much more detail about the mapping of the cropmarks from oblique photography and how this began to influence the patterns of later flights (, ). In fact, Riley also highlights a point about the flight process itself, suggesting that due to the banking of the aircraft required to capture the cropmarks, ‘much time was wasted while the aircraft was circling’ (ibid., ; see also Palmer this volume). However, this single highlight of a possible retrospective analysis of the flight process was not picked up in either The Emerging Past (Whimster ) or The Ancient Landscapes of the Yorkshire Wolds (Stoertz ), which both epitomise the style of recent aerial photographic publications by concentrating entirely on the imagery, and the mapping of the material from those images, with very little, if any, critique of the processes that have influenced the images used. This tendency can be further illustrated in Lincolnshire’s Archaeology from the Air (Bewley ), a volume that presents the results of a RCHME National Mapping Programme (NMP) project. In this publication those contributions dealing with the physical landscape and a series of period or class specific essays follow papers on the NMP. The paper entitled ‘The contribution of Aerial Survey: Understanding the Results’ (Carter ) is the final paper in the volume. It is something of a mantra in the aerial archaeological community that the results of aerial survey are routinely abused with little understanding of their character, and in this context the placement of

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FROM THE AIR

Ann Carter’s paper at the end of the volume could be read as symptomatic of not placing the issues of bias and subjectivity that pervade all survey to the fore. These issues should be to the vanguard; in our view explanation of the nature of the material should precede the more specific analysis (in this instance the editor placed the Carter paper at the back of the volume as a ‘rounding-off ’ piece (R.H. Bewley pers. comm.)). David Wilson’s second edition of the bible of air photographic interpretation, Air Photographic Interpretation for Archaeologists, again only hints at the issues involved. ‘When a specialist air photographer records an archaeological site in this way [oblique photography], the most important elements in the interpretation have taken place before he presses the button on the camera.’ (, ), suggesting that it has already been decided that the site is worthy of record, what angle it is to be viewed from etc., and that this leads to a very selective record. We would argue that this is entirely correct, but that there are other ‘elements’ involved in the pre-flight and in-flight interpretation process, and that, other than these statements by Wilson, little has been written about them. In contrast, the Aerial Archaeology E-mail Newsletter (ISSN 1479-6481), edited by Derek Edwards and circulated to about subscribers, the majority of whom are based in the United Kingdom and mainland Europe, provides a less formal forum for articulating exactly these issues (to subscribe to the newsletter, email: [email protected]). In fact, the various flight reports filed in this newsletter are often brief flight biographies, presented in an informal manner, but giving much more detail on those elements of the flight process that we wish to highlight. Here we find details of the various reasons for flights, why areas were chosen for inspection, what the original target(s) of the sorties was and whether this was successful or not. There are often descriptions of cropmarks viewed from the plane but not recorded for various reasons (e.g. weather closing in or controlled airspace), the use of ‘touchstone’ sites early in the cropmark season to try and predict the potential of areas to cropmark, and in many instances there are already interpretations of the cropmarks seen, such as whether something looks Roman or Neolithic depending on its character. Retrospective analysis of survey methodologies is a valid and vital process in understanding how our datasets come about. Reflexive practice in archaeological excavation and its interpretation is increasingly commonplace (i.e. Andrews et al. ; Hodder ), and aerial survey is an aspect of archaeological operations where such approaches are entirely appropriate. Although there has been a distinct trend in the last decade for the teaching institutions to reduce the amount of fieldwork that forms part of the archaeological learning experience, the understanding of the origins of archaeological data is of fundamental importance. This becomes paramount when the form of data acquisition is one that has necessarily restricted access due to costs and availability of the material required (such as aircraft), but which so obviously has a considerable impact on the perception of any given archaeological landscape. The flight biography, or at

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least a recognition of the various aspects of the flying process that contribute to the formation of the imagery that is interpreted and used in archaeology, should be a valid area for discussion and those who have no knowledge of this process need to be made aware so that they can make more informed interpretations of the data produced. What follows is a contribution to this facet of archaeological interpretation, where a specific flight is placed in the wider context of RCAHMS’ annual programme of summer reconnaissance and photography.

RCAHMS AERIAL SURVEY

Since , RCAHMS has pursued an annual programme of aerial reconnaissance during the summer, directed towards recording sites revealed by cropmarking, and less commonly, parchmarking. As a body of record, with responsibilities to provide national coverage, this survey has ranged across the cropmark-producing areas of Scotland, largely, but not exclusively, concentrated into the arable heartlands of the east coast running south from Aberdeen and the major river valleys of the south. This pattern of flying, during June, July and August is illustrated () by the GPS flight paths for the majority of the sorties from onwards, which are shown against the distribution of arable cultivation as mapped by the Macaulay Land Use Research Institute in . There is clearly an emphasis on these arable areas, which even after nearly years of reconnaissance are still producing new material. While there are issues regarding how representative is this focus on honeypot areas where high returns are guaranteed, as against working areas, such as pasture, where returns may be lower but which cumulatively serve to provide much more balanced site distributions (Cowley ), there is a clear need to continue working the arable areas. The strategy behind the annual cropmark survey season is conditioned by a number of factors, including the shared experience of over a quarter of a century of aerial survey, and soil moisture deficit data that has been shown to have an indicative value. In addition, there is a process of flying areas to evaluate conditions that begins at the start of June, and in a patchy year such as , continues through the summer. There is also a set of performance targets for aerial survey that have to be met. The aircraft used is a Cessna (four-seat) aircraft, with a photographer occupying the front, left seat (with an opening window) beside the pilot, and the flight director (this can be an archaeologist or architectural historian on an RCAHMS flight) occupying the left rear seat behind the photographer (). The flight director, sitting in the rather low rear seat, has no forward vision and has to do all observing out of the left side of the aircraft and, at a bit of a stretch, across the camera gear that usually occupies the fourth seat behind the pilot, out of the right side. Essentially, this set-up places a very strong emphasis on the left side of the aircraft, which has a significant impact on the placement of the aircraft

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FROM THE AIR

relative to what is being seen. The fourth seat behind the pilot is usually occupied by photographic gear, but it is also used for an additional observer, usually to provide training and experience. This practice also serves to offset some of the left-hand bias noted above, and there are examples of sites that would probably not have been found had this fourth seat not been occupied. As well as ensuring a wider view, two observers on the sortie provides for ‘on-site’ discussion and ‘peer review’, both processes that can help to offset the impact of lone-working where working practice may fossilise. However, this practice has implications for staff time and the weighting of the aircraft and its consequent fuel consumption. This set-up pertains to RCAHMS and we find it an effective way of working, but there is some variation in (equally valid) working practice across Europe. It is, for example, quite common for the seat beside the pilot to be occupied by an individual fulfilling the tasks of observer, navigator and photographer.

Map of the majority of GPS flight paths for RCAHMS summer sorties (-). Crown Copyright: RCAHMS

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A Cessna aircraft is not a large plane, and the working conditions for aerial surveyors are generally cramped, noisy and often cold when flying at ft with the window open. At the present time the flight director on an RCAHMS flight uses :, scale OS Landranger maps and a Garmin GPS to locate the aircraft accurately and note the location of any sites photographed. Sites are plotted on the map in coloured pencil and a note is made on a separate list of its form or immediate on-the-spot classification, the name of the nearest mapped farm, and a six figure grid reference if possible (all necessary to produce a primary log in advance of developing and printing of film, and in case the GPS fails). The aircraft is usually already in a relatively close circular flight path, entailing the left wing pointing more-or-less towards the ground, and ensuring the entire recording exercise is done at an angle. At the same time the photographer will have opened the window and begun photographing the site, creating a considerable air stream in the rear of the aircraft where the flight director is trying to record as well as continue examining the present site and look for others nearby. The photographer meanwhile, because of the configuration of the camera used, can no longer use the microphone on the headset to talk to the pilot or flight director, and must communicate how and where he wants the plane located using hand signals. When recording multiple sites, such as might occur during a particularly good cropmark season, the aircraft is constantly moving, swinging from side to side, changing from one close circle to another, and both the flight director and photographer have to concentrate on their tasks. This is not an

In flight over the Western Isles, Scotland: this view of the inside of a Cessna conveys something of its cramped interior. Crown Copyright: RCAHMS

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FROM THE AIR

environment conducive to careful consideration of the evidence being recorded, or even of the wider context of the individual photographed areas! In line with Civil Aviation Authority (CAA) guidelines the duration of the flight is limited to three hours (which must include an allowance for time waiting to land), conducted under Visual Flight Rules (VFR) that require good all-round visibility. There are specific controlled airspace zones in Scotland, which require careful planning to work within, if possible at all, and cover reasonably large percentages of Scotland. The summer of was warm and dry and a good cropmark year seemed likely. However, the patterns of the weather did not produce the crop stress required to induce widespread cropmarking. Indeed, the year was characterised by the patchiness of cropmark formation, varying from one river valley to another in a manner suggesting that very localised patterns of rainfall may have been exerting a significant influence. In this context it proved very difficult to predict the occurrence of cropmarks in any given area, and the process of flying large areas to assess the state of crops came to the fore; one such flight is described here.

The GPS flight path for sortie () with the distribution of arable ground, controlled air space and sites mentioned in the text. Crown Copyright: RCAHMS

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SORTIE , JULY – A POTTED BIOGRAPHY

The general route of this sortie was preconditioned by discussions in the office before the flight, which were informed by the need to provide a national coverage and the shared experience of staff involved in the aerial survey programme. This sortie was directed into the Earn and southern Perthshire, taking in areas that had been productive in the past. A routing northwards out of Edinburgh airport will take a small plane out of the Controlled Airspace (CTA)/Aerodrome Traffic Zone (ATZ)) at Kelty, to follow the line of the M to Kinross and thence across the east end of The Ochil Hills and into Strathearn (). With this sort of route it is likely that you end up over or near the extensive prehistoric complex at Forteviot (); the major elements of this, including the henges, were visible and were photographed as part of a trial of a digital camera. A westwards flight path was then chosen, placing the aircraft along the River Earn, more or less centered on the extent of the arable ground as marked in an indicative way (). The emphasis at this stage of the flight lay in looking at the variations in tone in the arable fields, but it rapidly became clear that it was only the palaeochannels on the flood plain that were showing in anything like a consistent way. Leadketty, another familiar and reliable face in the cropmark record, was also showing, and while it has been captured under better conditions, the absence of other material prompted a

An aerial view of part of the complex of sites at Forteviot in Perthshire. Crown Copyright: RCAHMS, E

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FROM THE AIR

series of images, again with the testing of the digital camera in mind. The extent of arable ground reduces further west in Strathearn (and has generally reduced significantly since the mapping of its extent in ) (), and here the increasing areas of improved pasture were examined for parchmarks, a particular interest for the flight director (DCC) but, unsurprisingly given the lack of cropmarking, there was a monotone of green fields. Investigating the more broken patches of arable and improved ground confirmed the non-productivity of this patch of ground, leading in time to Braco and the Roman fort of Ardoch. This was photographed, partly to enhance the colour holdings of the NMRS, but also, in small part, to ensure that the flight did at least return with something! The upper reaches of the Earn were examined, before returning along the north side of Strathearn towards Perth and then heading along the lower reaches of Strathmore, often a productive cropmark area, all the while confirming the lack of cropmark formation, and by this time having given up any hope of extracting anything from the pasture areas and concentrating on the honeypot areas of arable. The Roman fortlet at Cargill was again photographed: a very familiar face that can be relied on to show in even the most inauspicious conditions.

Aerial view of the pitted cursus monument and rig at Reedileys, in the Howe of Fife. This image was captured on a subsequent sortie due to a camera malfunction during sortie . Crown Copyright: RCAHMS, E

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The eastwards progress of the flight was limited by a bank of low cloud built up along the coast of Angus, and having seen so little to excite attention it was decided to return to Edinburgh, taking a slightly eastern route over Fife. There was a lot of activity at RAF Leuchars so it was not possible to do anything other than fly through an area where there were, as is usual in most years, at least some cropmarks showing. This route took us near the Howe of Fife, a large basin of freely draining soils blanketed in arable. On the eastern edge of this area a small, rather innocuous ring ditch was observed by the flight director and photographed, prompting a slight diversion to the west to examine the rest of the area. During this diversion, and in a sea of otherwise unresponsive fields, the cropmarks of a pitted cursus monument were discovered (). Neolithic cursus monuments are currently very much in the minds of the aerial survey team at RCAHMS because a volume on the Scottish examples is being prepared (Brophy & RCAHMS forthcoming), and at the instant of recognition of the features that made up this monument in the crop below, there was also a rush of excitement from the knowledge that this was a site that had not been previously recorded. This excitement was heightened by the fact that there was no ambiguity in the character of the monument although, as a pitted monument, the feature was not a bold one. Having photographed the site, a combination of time pressing and other flight activity around us sent us straight back to Edinburgh. From the point of leaving the controlled airspace to the north of the Forth, within the overall structure of the flight, the areas examined in general and those looked at in detail are predicated on an ongoing series of value judgments that reflect the patterns that caught the eye of the flight crew and the decisions based on them. As such the flight was essentially a highly subjective process, but one that is informed by past experience and the ongoing stimulus of working a piece of ground, attempting to identify the patterns that produce a return, and minimising input to unproductive searches. In the context of the discovery of the ‘new’ pitted cursus at Reedieleys (Brophy & RCAHMS forthcoming; Cowley & Gilmour ), the fact that a small innocuous ring ditch caught the eye of the flight crew and resulted in a diversion into the area, highlights the accidental nature of many such discoveries, mirroring Crawford’s experience nearly three-quarters of a century earlier. The changes in the state of mind during this sortie, ranging between an initial optimism, to a growing sense of disappointment, through to the surprise and excitement of discovery at the end of the flight, encapsulate aspects of many programmes of ongoing survey. The mundane and routine predominate, but every so often there is the rush of excitement in discovery and a heightened engagement in the process of interpretation and analysis, something that is not limited to the survey process alone, but can occur during, for example, routine cataloguing. The patterns illustrated in this sortie represent, in microcosm, many of the vagaries of discovery in summer flying as a whole. It also reinforces the importance of maintaining programmes of prospective flying, because major

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discrete monuments such Reedieleys will continue to emerge, while the ongoing recording of other elements in the jigsaw will continue to enhance our understanding of patterns in the wider landscape.

CONCLUSION

In describing the context and circumstances of the discovery of a cursus monument in Fife, this paper has attempted to shed some light on aspects of the processes involved in producing a resource that is often used without questioning the impact that its methodologies, preconceptions and other inherent and unspoken biases, may have on the character of the data. Source criticism is vital to make proper use of data, and has a very significant role to play in the development of methodologies. In this process, survey practitioners must maintain a self-critical perspective, in which the reviewing of flights and the data produced, plays a central role in developing future practice, ensuring it does not stagnate, simply reinforcing known patterns in our data, but continues to expand the boundaries of our knowledge. The notion of biographies of flight, including pre-flight planning, the mechanisms of a sortie and the experience of the flight, may be a useful one for both presenting and explaining the survey process to non-fliers, but also for providing a mechanism for the self critical assessment of our own procedures: an autobiography if you like. This process should also be extended to the critical assessment of the products and progress of the national bodies of survey (RCHME/English Heritage, RCAHMS and the RCAHMW), as well as regional fliers (see for example Oakey this volume). This type of analytical approach is vital to maintaining best practice and developing future practice that does not simply reinforce what we have already learnt. This is especially pertinent where the substantial and vitally important datasets that are the national collections of aerial photographs are being produced by a handful of individuals. These individuals need to maintain a self-critical approach that is open to the criticisms of end-users as part of a mature dialogue.

ACKNOWLEDGEMENTS

We are grateful to various colleagues at RCAHMS, Bob Bewley and Rog Palmer for comments on the text, and to Georgina Brown for producing the two maps (, ).

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Andrews, G., Barrett, J.C. & Lewis, J.S.C. Interpretation not record: the practice of archaeology, Antiquity , No. , -

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Brophy, K. Thinking and doing aerial photography, AARGnews , -Brophy, K. & RCAHMS forthcoming The Neolithic Cursus Monuments of Scotland, RCAHMS/Society

of Antiquaries of ScotlandCarter, A. The Contribution of Aerial Survey: Understanding the Results, in: Bewley, R.H. (ed.)

Lincolnshire’s Archaeology from the Air, Occasional Papers in Lincolnshire History and Archaeology, Volume , -

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of SheffieldSt Joseph, J.K.S. The Uses of Air Photography (second edition), John BakerStoertz, C. Ancient Landscapes of the Yorkshire Wolds: aerial photographic transcription and analysis,

RCHMEWhimster, R. The Emerging Past: air photography and the Buried Landscape, RCHMEWilson, D.R. Air Photo Interpretation for Archaeologists (second edition), Tempus

Some observations on the nature of aerial survey

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